The present invention relates to a process for the excitation of a sample for NMR tomography, wherein the sample is exposed to a homogenous magnetic field and in addition to a selection gradient, and is excited by a 90.degree. selection pulse, whereafter the selection gradient is replaced by a time-restricted phase coding gradient and a read gradient, the selection gradient, phase coding gradient, and read gradient being perpendicular to each other respectively in pairs, and wherein lastly the sample, after the end of the phase coding gradient, is irradiated during the prevailing read gradient with a sequence of 180.degree. pulses, whereby measurable core induction signals in the form of the so-called spin echos are generated.
The generation and measurement of core resonance signals after the influencing of the sample by a time-restricted phase coding gradient which is supplied before switching on the read gradient leads by the use of a two-dimensional Fourier transformation to the generation of cross-sectional images, for which reason this process is also named 2DFT. As was stated above, it is conceivable in principle to use for the generation of core induction signals the spin echo pulse sequences which are generally employed in NMR spectrometry, such as particularly the Carr-Purcell pulse sequence or the Carr-Purcell-Gill-Meiboom pulse sequence. These pulse sequences, after common excitation, make possible the generation of a plurality of echo signals which can be added up to improve the signal-noise ratio or can be used to determine the spin-spin relaxation time T.sub.2. But it has been found that when employing the 2DFT method the previously known pulse sequences are only usable to a limited extent, since with the growing number of echos, artefacts in the form of images appear to an increasing extent, which are reflected with respect to the undisturbed image on an axis which is parallel to the direction of the read gradient. Such artefacts will hereinafter be named mirror images.
Since on the other hand for example the exact determination of T.sub.2 or an analysis according to various T.sub.2 contributions during a multi-exponential spin-spin relaxation is only practicable with the aid of many spin echos, the 2DFT process could not previously be used for such investigations which are of great significance in diagnostic medicine, although it has the advantage that good images are supplied even with relatively low homogeneity of the magnetic field, so that this process would be especially well suited, inter alia, for simple installations from this point of view.
Accordingly it is the object of the invention to improve the process of the type described initially so that when using spin echo pulse sequences, no mirror images will arise.
This object is achieved according to the invention in that the sample, in the course of a spin echo pulse sequence, is subjected to the influence of the phase coding gradient in such a manner that the dephasing effected between each two 180.degree. pulses by the phase coding gradient is twice as great as the dephasing which is effected before the first 180.degree. pulse by the phase coding gradient.
It is known in NMR tomography as well as in NMR spectroscopy that when using a spin echo pulse sequence, the rephasing condition for the read gradient must be fulfilled. This is most simply achieved in that with constantly occurring read gradients the time interval of two adjacent 180.degree. pulses is chosen so as to be twice as great as the interval between the beginning of the dephasing by the read gradient and the first 180.degree. pulse.
The process according to the invention has the result that the rephasing conditions are fulfilled not only for the read gradient but also for the phase coding gradient. However the measuring process is only carried out in the presence of the read gradient, whereas the phase coding gradient is switched so that by means of this gradient constant dephasing during the period of each measuring process is effected which is identical for all the measuring processes within one spin echo pulse sequence. This is attained by repeated switching on of the phase coding gradient between each pair of 180.degree. pulses in any of the ways which will be described below.
According to a first embodiment of the invention, the phase coding gradient is switched on before the beginning of the first 180.degree. pulse for the first time, whereby a certain dephasing is caused, and then switched on again between each two 180.degree. pulses, but after the ending of the measuring process for each spin echo, so that the dephasing caused thereby is twice as large as the dephasing caused during the first action of the phase coding gradient before the first 180.degree. pulse.
According to a second embodiment of this invention, the phase coding gradient is switched on for the first time before the beginning of the first 180.degree. pulse, whereby a certain dephasing is effected, and subsequently again between each pair of 180.degree. pulses, but before the beginning of the measuring process for each spin echo, so that the dephasing caused thereby is twice as large as the dephasing caused during the first action of the phase coding gradient before the first 180.degree. pulse.
According to a third embodiment of the invention, the phase coding gradient is not switched on at all before the first 180.degree. pulse, but is switched on for the first time after the first 180.degree. pulse, in such a way that it is at first switched on between each two 180.degree. pulses before the beginning of the measuring process, so that a certain dephasing is caused, and subsequently after the end of the measuring process, but before the beginning of the next 180.degree. pulse it is switched on again with reversed sign so that this second action of the gradient again cancels the dephasing effected by the first action.
The process according to the invention makes possible in particular the use of a conventional Carr-Purcell-Gill-Meiboom pulse sequence for NMR tomography using the 2DFT process. By adhering to the rephasing condition for the phase coding gradient as well, the effects which lead to the formation of mirror images are cancelled out. Thus the phase coding gradient can be switched on without any difficulties additionally to the read gradient, because when using the 2DFT process the effect of the two gradients is independent of each other.